1. Field of the Invention
The present invention relates to a nitride based semiconductor device, and more particularly to a high efficiency nitride based semiconductor device having improved internal quantum efficiency, operation voltage, and reverse voltage properties.
2. Description of the Related Art
Generally, nitride based semiconductor devices have been widely used in green light or blue light emitting diodes (LEDs) or laser diodes (LDs) provided as a light source for use in full color displays, image scanners, a variety of signaling systems and optical communication instruments. Such nitride based semiconductor devices include an active layer of a single quantum well (SQW) structure or a multi quantum well (MQW) structure disposed between n- and p-type nitride semiconductor layers, and the active layer produces and emits light according to the principles of recombination between electrons and holes.
Light efficiency of the nitride based semiconductor devices is determined by the probability of recombination between electrons and holes in the active layer, i.e., internal quantum efficiency. Schemes for improving the internal quantum efficiency are largely focused on improving the structure of the active layer or increasing effective mass of carriers.
As such a conventional method for the above-mentioned purpose, reference “ELECTRON DEVICE LETTERS, Vol. 23, No. 3 Mar. 2002, p 130” by IEEE (Institute of Electrical and Electronics Engineers, Inc) has proposed use of a charge asymmetric resonance tunneling structure made of an InGaN/GaN layer below the active layer of the multi quantum well structure. According to the above-mentioned reference, it is stated that luminous efficiency of the nitride based semiconductor device can be improved by introduction of a 50 nm thick InGaN layer and a 1 nm thick GaN layer so as to inject electrons accumulated in the InGaN layer into the active layer by tunneling, leading to decrease of operation current and voltage. In this manner, the InGaN layer and GaN layer increase effective mass of electrons which is usually lower than that of holes by taking advantage of the tunneling effect and thereby can effectively serve as an electron emitting layer which increases the probability of carrier capture in the active layer.
However, the above-mentioned method still suffers from a problem of lowering recombination efficiency between electrons and holes due to piezoelectric field. That is, difference of lattice constants between the active layer and adjacent clad layer produces stress that in turn is applied to the active layer thus forming the piezoelectric field.
Referring to an energy band diagram of the active layer under conditions of no stress, as shown in FIG. 1a, a wave function of electrons and holes is practically symmetrical. However, when compressive stress or tensile stress acts due to the difference of lattice constants between the active layer and clad layer, as shown in FIGS. 1b and 1c, the piezoelectric field is formed as represented by the arrows and a phenomenon occurs wherein the distance between wave functions of electrons and holes in the active layer become more distant. Therefore, even though the effective mass of carrier injected increases, the recombination probability between electrons and holes does not substantially increase, thus leading to deterioration of luminous efficiency of the optical device. Further, because of increased current due to increased distance between wave functions, emitted light is shifted to a short wavelength region.
There remains a need in the art for a novel method of solving problems associated with deterioration of luminous efficiency and wavelength shift of emitted light due to the piezoelectric field.